Business and consumers use a wide array of wireless devices, including cell phones, wireless local area network (LAN) cards, global positioning system (GPS) devices, electronic organizers equipped with wireless modems, and the like. The increased demand for wireless communication devices has created a corresponding demand for technical improvements to such devices. Generally speaking, wireless system designers attempt to minimize the cost of conventional radio receivers while improving the performance of such devices. Performance improvements include, among other things, lower power consumption, greater range, increased receiver sensitivity, lower bit error rates (BER), higher transmission rates, and the like.
Signal fading due to variations in channel characteristics is a major factor limiting the performance of modern mobile wireless communication systems. To compensate for signal fading, many modern code division multiple access (CDMA) networks use diversity techniques to transmit multiple copies of a signal over a channel to a mobile station. In the mobile station, a RAKE receiver uses multiple baseband correlators to individually process several signal multipath components. The correlator outputs are then combined to achieve improved performance.
A RAKE receiver comprises L fingers, where each of the L fingers contains a baseband correlator that processes one of the multipath components. A typical spread spectrum receiver comprises a code phase acquisition circuit that detects multipath components of a transmitted signal and assigns (or allocates) each of the strongest multipath component signals to one of the L RAKE fingers.
However, the channel delays associated with the multipath components are non-stationary. As a result, the multipath components allocated to the RAKE fingers may disappear as the mobile station (e.g., cell phone) moves and the channel delay profile changes. Thus, it is necessary to deassign RAKE fingers once their multipath components are lost, to continuously look for new multipath components, and to assign the new multipath components to deassigned RAKE fingers.
A system for assigning (allocating) and deassigning (deallocating) RAKE fingers is discussed in “Grouped RAKE Finger Management Principle for Wideband CDMA”, B. N. Vejlgaard et al., IEEE 2000. However, the apparatus disclosed in the Vejlgaard et al. disclosure only takes finger power into account when making assignment decisions. A very brief fast fading of a multipath component may cause the multipath component to be unnecessarily deassigned from a RAKE receiver finger. When the fade ends after a very brief period, the recovered multipath component is reassigned to the RAKE receiver finger again. Also, the prior art Vejlgaard et al. reference is wasteful of RAKE receiver fingers in that it assigns fingers by groups of three that do not move independently. This increases the number of fingers required and also decreases the resolvability of the RAKE receiver fingers.
Therefore, there is a need in the art for improved RAKE receivers. More particularly, there is a need for improved methods and apparatuses for managing the assignment and deassignment of fingers in a RAKE receiver.